A wide variety of heart valve prostheses have been developed which operate hemodynamically, in conjunction with the pumping action of the heart, to take the place of a defective natural valve. These valves have generally been designed to function with valve members in the form of a single occluder, a pair of occluders or leaflets or even three or more occluders; such occluders pivot along eccentric axes (or both pivot and translate) to open and close a central blood flow passageway through a generally annular valve body within which the occluders are usually appropriately supported.
U.S. Pat. No. 4,451,937 (Jun. 5, 1984) discloses an early heart valve design wherein arcuate depressions in flat sidewall sections of a Valve body guide valve members having ears extending from their lateral edges that are received in such depressions.
U.S. Pat. No. 4,689,046 (Aug. 25, 1987) discloses a bileaflet heart valve having a pair of flat leaflets with ears of generally trapezoidal configuration extending from the flat lateral surfaces thereof. The ears have flat end faces and are received in diametrically opposed recesses in the valve body having facing flat end surfaces; the recesses are shaped so that the ears are rockingly engaged therein by tapered recess guide wall surfaces of arcuate configuration.
U.S. Pat. No. 5,123,920 (Jun. 23, 1992) discloses a bileaflet heart valve having curved leaflets with bulbous downstream sections having a pivot construction wherein notches are formed in the outflow surfaces of thickened portions of the pair of leaflets, which notches engage complementary surfaces on pivot projections that extend radially inward from diametrically opposite locations on the valve body sidewall.
U.S. Pat. No. 5,137,532 (Aug. 11, 1992) discloses bileaflet heart valves having pivot arrangements which allow the leaflets to assume an orientation substantially parallel to the centerline through the valve in their open position in a valve body which is elongated in axial length relative to bileaflet valves of earlier design wherein designers generally attempted to minimize the length of the blood flow path through the valve body, because the valve was felt to be confining. In one embodiment, camming surfaces provided on the leaflets engage appropriately located projections extending radially inward from the valve body sidewall, and the upstream displacement of the leaflets which occurs upon the reversal of blood flow causes prompt pivoting of the leaflets toward the closed positions.
U.S. Pat. No. 5,152,785 (Oct. 6, 1992) and U.S. No. 5,192,309 (Mar. 9, 1993) show heart valves which are generally similar to that last mentioned. The '309 patent illustrates valves having an alternative construction wherein inclined camming surfaces are provided on projections located at the upstream edge of the valve body, which are engaged by the upstream edges of the respective leaflets to create a camming action. Guidance for determining the path of the leaflets is also provided by cylindrical lateral ears that translate in slots formed in flat sidewall portions of the valve body.
U.S. Pat. No. 5,350,421 (Sep. 27, 1994) is similar to the '309 patent and specifically illustrates a construction that is responsible for prerotation of the leaflets occurring at the end of the downstream flow of blood through the valve just prior to its reversal.
U.S. Pat. No. 5,314,467 (May 24, 1994) discloses a bileaflet heart valve wherein leaflets of composite curvature are supported by laterally extending elongated ears which are received in recesses formed in diametrically opposed flat wall sections of the interior surface of a valve body that is formed with a flared outflow seat region against which the leaflet downstream edges seat. The recesses each have a serpentine guide wall along the upstream edge thereof. The combination of it and a second downstream wall creates a sequence of rotational and then translational movement of the leaflets as they pivot from the open position to the closed position.
More recently, attention has also begun to be given to trileaflet valves, and the study of blood flow through such multiple leaflet valves has convinced many investigators that it is very important that emphasis should be given to achieving designs with minimum turbulence and minimum pressure drop. It was generally believed that the shorter the axial length of a valve body was, the less would be the resistance to blood flow through the critical region of the valve, because the valve body was of course the region of greatest constriction. Many patented valve designs also concentrated on the shape and the placement of the occluders to minimize pressure drop and turbulence.
A number of U.S. patents, such as U.S. Pat. Nos. 4,363,142, 4,328,592, 5,178,632 and 5,171,623 illustrate heart valves having relatively short valve bodies of generally circular cross-section, some of which have rounded or radially outwardly flared upstream and downstream ends. U.S. Pat. No. 5,078,739 shows a heart valve having a sloping entrance end wherein the leaflets are mounted external of the valve body via resilient hinges embedded in the downstream end surface of the valve body. U.S. Pat. No. 4,775,378 shows a heart valve having a single occluder with a shallow S-shaped curvature that is alleged to promote the formation of a stable closed vortex on the suction side of the occluder; it is employed in combination with a valve body having a circular cross-section passageway that is continuously and increasingly constricted, i.e. its diameter decreasing, in the downstream direction. U.S. Pat. No. 4,846,830 discloses a bileaflet valve having a similar valve body wherein a pair of curved leaflets are employed which are arranged to create a venturi tube nozzle in the direction of downstream flow which is alleged to avoid vortex formation. U.S. Pat. No. 4,995,881 shows a valve having a similarly sloping entrance in combination with a pair of leaflets that are curved in the downstream direction so as to define a nozzle-shaped passage centrally between the two leaflets when they are in their open position orientation.
The more that such mechanical prosthetic valves have been studied, the more that investigators have concluded that the ideal prosthetic valve simply does not yet exist. From a materials standpoint, pyrolytic carbon has been determined to be adequately nonthrombogenic; as a result, the problem of combatting thrombosis in mechanical valves is presently felt to lie in preventing excess turbulence, high shear stresses and local regions of stasis. Blood is a very delicate tissue, and even minor abuses caused by turbulence and high shear stress can cause either thrombosis or emboli generation at local regions of stagnation. Therefore, it is felt that future improvement in the characteristic of thromboresistance in mechanical valves will likely be attained through the achievement of smooth, nonturbulent flow and the absence of stasis.
The search continues for improved mechanical heart valve prostheses that provide passageways through which blood will flow freely and with a minimum of drag in the open position, which will close quickly upon the occurrence of backflow to minimize regurgitation of blood, and which can be efficiently manufactured and assembled. Accordingly, new valve designs incorporating such features have continued to be sought.